I don't know, there's still fanservice, just no panty shots.
It's some kind of self censorship of the panties. It bugs me not because I wanna see their panties, but because when they're wearing such short skirts and flipping around and stuff, it starts to actually stand out that you DON'T see their panties. Like even if it wasn't overtly sexualised, they could at least make it so you glimpse it in a manner that makes sense, rather than make it seem as though their legs are imposibly long and enter some kind of infinite dimension within the space of the skirt, like they never meet.
And also, say with something like Gurren Lagann, it just enforces the possibilty that the girl in question doesn't have any underwear, and that's why they show nothing.